1. Field of the Invention
The present invention relates to an optical filter applicable to gain equalizers for equalizing optical signals in optical amplifiers, and the like.
2. Related Background Art
An optical amplifier includes an amplification optical waveguide doped with a fluorescent material which is excitable with pumping light, and a pumping light source for supplying the pumping light to the optical waveguide; and is disposed in a repeater station in an optical transmission system or the like. In particular, it is important for the optical amplifier employed in a wavelength division multiplexing transmission system (WDM transmission system) transmitting a plurality of wavelengths of optical signals to not only collectively amplify a plurality of wavelengths of optical signals with gains identical to each other, but also output the individual optical signals with their power attaining a predetermined target value. Therefore, in order to equalize the amplification gain of optical signals in such an optical amplifier, an optical filter having a loss spectrum with a form identical to that of the gain spectrum in the signal wavelength band has been in use.
For example, a technique aimed at flattening the gain of an optical amplifier by use of an optical filter employing a Mach-Zehnder interferometer is disclosed in the document 1, K. Inoue, et al., "Tunable Gain Equalization Using a Mach-Zehnder Optical Filter in Multistage Fiber Amplifiers," IEEE Photonics Technology letters, Vol. 3, No. 8, pp. 718-720 (1991). Also, an optical filter in which two optical filters each having the structure described in the above-mentioned document 1 are cascaded to each other is disclosed in the document 2, H. Toba, et al., "Demonstration of Optical FDM Based Self-Healing Ring Network Employing Arrayed-Waveguide-Grating ADM Filters and EDFAs," Proceedings of ECOC'94, pp. 263-266 (1994). Further, an optical filter comprising a Faraday rotator adapted to alter the amount of rotation of polarizing azimuth of light, a birefringent plate, two birefringent wedge-shaped members, and a lens system is disclosed in the document 3, T. Naito, et al., "Active Gain Slope Compensation in Large-Capacity, Long-Haul WDM Transmission System," Proceedings of OAA'98, WC5, pp. 36-39 (1999).